Many transmission system circuits require AC coupling to the outside world to eliminate problems of DC incompatibility between external signals from outside the circuit and reference points of various nodes within the circuits.
An AC coupler, in addition to blocking DC signals, preferably admits AC signals unaffected in a frequency band of interest (though a constant attenuation in the frequency band may be tolerated). The transfer function of an AC coupler is, therefore, often chosen to be a high-pass type with one or more zeroes at the origin. In the instance of telephone circuits, the band of interest begins at 300 Hz. Accordingly, AC couplers in telephone circuits typically have poles at much less than 300 Hz (usually close to 1 Hz), in order to have a negligible error in the band of interest.
The availability of an AC coupler which conforms with the specifications of different telephone companies without the use of external components is an important condition to the development of new speech circuits which are cost-effective and reliable. Current art solutions provide for the use of discrete components external to the AC coupler in conjunction with monolithically integrated circuits.
For example, an AC coupler having a transfer function with a pole at a frequency on the order of one Hertz is typically implemented with an external capacitor of some tens of .mu.F and an internal resistance on the order of tens of kiloohms The use of such external components lowers the degree of reliability and raises costs. Additionally, the AC coupling in some cases is implemented between internal blocks of the integrated circuit. In such a case, besides the external component, two additional pins are also required for connection to the AC coupler.
In any case, the presence of an external capacitor involves a further disadvantage, in that upon turning on the circuit, a charge current of at least a few milliamperes is required to attain steady-state voltage at the external capacitor within a short time.